It is well known that this type of blood flowmeter causes the blood flow in the magnetic field to induce a voltage on the detector electrodes on the basis of Faraday's law. Usually the transformer component is superposed on the detected signals due to the occurrence of the cross field in the lead lines of the detector electrodes by the exciter coil. Also ECG signals directly induced on the detector electrodes are mixed therein independently of the excitation, as shown in FIG. 1. Such an ECG signal has a lower frequency component as compared with the carrier of the blood flow signal, and, therefore, can be eliminated through a high-pass filter. But, the influence on the blood flow signal is inevitable when a large amount of reduction is intended. From this point of view, if the blood flow signal is synchronous-detected in addition to a proper high-pass filtering, the ECG signal may be substantially eliminated, since it is asynchronous with the carrier of the blood flow signal and has the lower frequency component as compared with the carrier. However, in this case, when the ECG signal level is so high as to saturate in the amplifier of the preceding stage, the detected DC blood flow signal vanishes as the case may be and furthermore the transformer component steadily superposed may vanish together with the ECG signal.
On the other hand, circuits are known for only eliminating the transformer component from the blood flow signal. Therefore, in any method where only the ECG signal is effectively eliminated, a pure blood flow signal will be obtained.